1. Field of the Invention
The invention relates to a gallium nitride substrate and an optical device using the gallium nitride substrate.
2. Description of the Related Art
GaN-based semiconductor crystals such as gallium nitride (GaN) has attracted attention as a material of optical devices such as light emitting diode (LED) which emits high-intensity blue light or long-life laser diode (LD) which emits blue light.
Bulk crystal growth of the GaN-based semiconductor crystals is difficult, and accordingly, it is difficult to produce a large single crystal GaN with high quality. However, in recent years, a method of manufacturing a GaN-based semiconductor crystal has been proposed, using a DEEP (Dislocation Elimination by the Epi-growth with Inverted-Pyramidal Pits) method or a VAS (Void-Assisted Separation) method, etc., and also using GaN free-standing substrate in which a GaN single crystal is grown on a heterogeneous substrate by a HVPE (Hydride Vapor Phase Epitaxy) method.
In the DEEP method, a patterned mask of SiN, etc., is formed on a GaAs substrate which is removable by etching, a GaN layer is the formed thereon, plural pits surrounded by facet planes are intentionally formed on a crystal surface and dislocations are accumulated at a bottom of the pits to reduce dislocation in other regions. The GaAs substrate is then removed, thereby obtaining a GaN free-standing substrate with reduced dislocation (see, e.g., JP-A-2003-165799).
In the VAS method, a GaN layer is grown on a substrate of sapphire, etc., via a GaN substrate with voids and a TiN thin film having a mesh structure, thereby allowing separation of the GaN substrate and reduction of dislocation at the same time (see, e.g., JP-A-2004-269313).
The GaN free-standing substrate obtained by the above-mentioned methods is flattened by grinding and polishing front and back surfaces of a substrate epitaxially grown by the HVPE method. Subsequently, an outer periphery of the substrate is shaped in order to have a circular shape with a given diameter. Then, after removing processing strain by wet-etching, etc., the substrate is cleaned and a GaN mirror wafer is thus obtained.
A known method of polishing the GaN substrate is, e.g., disclosed in JP-A-2001-322899. In JP-A-2001-322899, after the GaN substrate is fixed to a substrate-attaching board using a wax, both front and back surfaces of the GaN substrate are polished by loose abrasive supplied onto the surface plate. Diamond is used as the loose abrasive by taking into consideration hardness of the GaN substrate.